Optical cable

Details Optical cable Optical cable

2000-03-13

2001-04-03

Ullah, Akm E. (Department: 2874)

Optical waveguides

Optical transmission cable

Loose tube type

Reexamination Certificate

active

06212321

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical cable having an S-Z type helical groove for containing stacked optical fiber ribbons.
2. Related Background Art
As a technique in such a field, Japanese Patent Application Laid-Open No. HEI 8-211264has been known. This publication discloses a technique in which the width and depth of an S-Z type helical groove provided in a chamber element are made greater than the diagonal of a stack of optical fiber ribbons.
SUMMARY OF THE INVENTION
The above-mentioned conventional optical cable has the following problems. Namely, while it is preferred that the optical fiber ribbons be contained within the S-Z type helical groove from the viewpoint of securing a long-term reliability, it is necessary to keep the optical fiber ribbons from collapsing within the groove from the viewpoint of preventing transmission loss from increasing due to cabling. In particular, it has been known that the stack of optical fiber ribbons generally tends to move toward the opening of the groove in the vicinity of reverse portions of the groove in order to reduce the curvature of the optical fiber ribbons. Therefore, if the groove width is broader than necessary in the conventional optical cable, then the stack may rotate within the groove, whereby the form of the stack may collapse, which may cause transmission loss to increase.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide, in particular, an optical cable which reliably inhibits, over the whole length of a stack of optical fiber ribbons contained in a helical groove of a chamber element, the stack from collapsing.
For overcoming the above-mentioned problems, the optical cable in accordance with the present invention is an optical cable comprising a tension member disposed at a center thereof, an elongated chamber element having at least one S-Z type helical groove whose direction of strand on an outer periphery reverses periodically, and a plurality of optical fiber ribbons stacked within the helical groove of the chamber element; wherein a reversal angle from one reverse portion to a next reverse portion in the helical groove is at least 180 degrees; and wherein, letting W, T, and n be the width and thickness of each optical fiber ribbon and the number of stacked optical fiber ribbons, respectively, and a and b be the width and depth of the helical groove, respectively, at least the reverse portions of the helical groove have a cross-sectional form satisfying:
nT<a
≦{square root over (
W
2
+L +(
nT
+L )
2
+L )}  (1)
W<b  (2)
whereas the remaining portion of the helical groove has a cross-sectional form satisfying:
{square root over (
W
2
+L +(
nT
+L )
2
+L )}<min(
a,b
)  (3)
In this optical cable, the twist of the stack itself is peaked in the reverse portions of the helical groove, so that the form of the stack is most likely to collapse there. Hence, the width of the helical groove is made smaller than the length of the diagonal of the stack at least in the reverse portions, whereby the stack is reliably prevented from collapsing when moving from the groove bottom toward the groove opening. In the portion where the stack is hard to collapse, on the other hand, the width and depth of the helical groove are made greater than the length of the diagonal of the stack, so that the stack is contained within the helical groove with a margin. Namely, in this portion, the relative movement of the helical groove with respect to the stack is made smooth. Therefore, the stack can be contained in a stable state over the whole length of the helical groove, whereby transmission loss is appropriately inhibited from increasing due to the collapsing of the stack.
Alternatively, the optical cable in accordance with the present invention is an optical cable comprising a tension member disposed at a center thereof, an elongated chamber element having at least one S-Z type helical groove whose direction of strand on an outer periphery reverses periodically, and a plurality of optical fiber ribbons stacked within the helical groove of the chamber element; wherein a reversal angle from one reverse portion to a next reverse portion in the helical groove is at least 180 degrees; and wherein, letting W, T, and n be the width and thickness of each optical fiber ribbon and the number of stacked optical fiber ribbons, respectively, and a and b be the width and depth of the helical groove, respectively, the helical groove has a cross-sectional form satisfying the above-mentioned expressions (1) and (2) in an area where a rotational angle with reference to a transit center portion located between neighboring reverse portions is at least 90 degrees, whereas the remaining portion of the helical groove has a cross-sectional form satisfying the above-mentioned expression (3).
In this optical cable, the twist of the stack itself is peaked in the reverse portions of the helical groove, so that the form of the stack is most likely to collapse there. Hence, the width of the helical groove is made smaller than the length of the diagonal of the stack in an area where the rotational angle with reference to a transit center portion located between neighboring reverse portions is at least 90 degrees, i.e., in the area where the stack is likely to collapse within the helical groove, whereby the stack is reliably prevented from collapsing when moving from the groove bottom toward the groove opening. In the portion where the stack is hard to collapse, on the other hand, the width and depth of the helical groove are made greater than the length of the diagonal of the stack, so that the stack is contained within the helical groove with a margin. Namely, in this portion, the relative movement of the helical groove with respect to the stack is made smooth. Therefore, the stack can be contained in a stable state over the whole length of the helical groove, whereby transmission loss is appropriately inhibited from increasing due to the collapsing of the stack.
Here, it is preferred that the cross-sectional form of the helical groove of the chamber element have a bottom portion which is substantially shaped like an arc. As a consequence, it becomes easier for the stack within the helical groove to shift to a state having the least twist.
Letting r be the radius of the arc of the bottom portion of this helical groove, it is preferred that
W
2
+
(
nT
)
2
2
≤
r
(
4
)
be satisfied.
Preferably, a stack constituted by a plurality of the optical fiber ribbons is contained in the helical groove with a ribbon plane thereof facing the bottom face of the helical groove in the vicinity of the transit center portion located between neighboring reverse portions, whereas the stack is contained in the helical groove with a ribbon side face thereof facing the bottom face of the helical groove in the vicinity of the reverse portions. When the stack is contained within the helical groove in such a state, the transmission loss of coated optical fibers can be suppressed as much as possible.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.


REFERENCES:
patent: 5638478 (1997-06-01), Iwakura et al.
patent: 8-211264 (1996-08-01), None
patent: 9-049949 (1997-02-01), None

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